Patrick Wellenius

Room temperature pulsed laser deposited indium gallium zinc oxide channel based transparent thin film transistors

Indium gallium zinc oxide deposited by pulsed laser deposition at room temperature was used as a channel layer to fabricate transparent thin film transistors with good electrical characteristics: field effect mobility of 11cm2V−1s−1 and subthreshold voltage swing of 0.20V∕decade. By varying the oxygen partial pressure during deposition the conductivity of the channel was controlled to give a low off-current of ∼10pA and a drain current on/off ratio of ∼5×107. Changing the channel layer thickness was a viable way to vary the threshold voltage. The effect of the gate dielectric on the electrical behavior was also explored.

Transparent indium gallium zinc oxide transistor based floating gate memory with platinum nanoparticles in the gate dielectric

A transparent memory device has been developed based on an indium gallium zinc oxide thin film transistor by incorporating platinum nanoparticles in the gate dielectric stack as the charge storage medium. The transfer characteristics of the device show a large clockwise hysteresis due to electron trapping and are attributed to the platinum nanoparticles. Effect of the gate bias stress (program voltage) magnitude, duration, and polarity on the memory window characteristics has been studied. Charge retention measurements were carried out and a loss of less than 25% of the trapped elec-trons was observed over 104 s indicating promising application as nonvolatile memory.

Fast All-Transparent Integrated Circuits Based on Indium Gallium Zinc Oxide Thin-Film Transistors

We describe the fabrication and characterization of visible transparent small-scale indium gallium zinc oxide (IGZO) integrated circuits. The IGZO channel and indium tin oxide (ITO) contacts and interconnects were pulsed laser deposited at room temperature. Low-temperature (200 °C ) atomic-layer-deposited Al2O3 was used as the gate dielectric in bottom-gated thin-film transistors with field-effect mobility near 15 cm2/V·s. Logic inverters and ring oscillators were fabricated and characterized, with operations at frequencies as high as 2.1 MHz, corresponding to a propagation delay of less than 48 ns/stage with a supply voltage of 25 V. To the best of our knowledge, these are the fastest all-transparent oxide semiconductor circuits reported to date.

Bright, low voltage europium doped gallium oxide thin film electroluminescent devices

Europium doped gallium oxide thin film electroluminescent devices with bright, red emission (611nm) and relatively low threshold voltages of 60V were produced using pulsed laser deposition. The use of transparent conducting electrodes of amorphous InGaZnO on transparent aluminum titanium oxide/indium tin oxide/7059 Corning glass substrates resulted in a device that is transparent throughout the visible spectrum. At 100V, with 1kHz excitation, the luminance was 221cd∕m2. The Sawyer-Tower circuit analysis and time dependent emission measurements suggest that the charge trapping at the aluminum titanium oxide/Ga2O3:Eu interface plays an important role in producing efficient emission.

Optical characterization of Eu-doped β-Ga2O3 thin films

Europium-doped β-Ga2O3 thin films were grown on double-side polished c-axis (0001) sapphire substrates by pulsed laser deposition at 850°C. Transmission measurements of the films revealed a sharp band edge with a band gap at 5.0eV. The films exhibited intense red emission at 611nm (2.03eV) due to the transitions from D05 to F27 levels in europium, with intensities that increased with the concentration of europium. Time-resolved photoluminescence measurements revealed a temperature-insensitive lifetime of 1.4ms, which is much longer than the lifetimes of europium luminescence observed in GaN hosts.

High performance transparent thin film transistors based on indium gallium zinc oxide as the channel material

The fabrication of high performance amorphous indium gallium zinc oxide (IGZO) transparent thin film transistors (TTFT) and their bias stress stability is presented. N-channel enhancement mode devices were fabricated with an extracted field effect mobility of ~ 11-15 cm2 V-1s-1, on/off current ratios > 107, subthreshold gate voltage swing of 0.20-0.25 V/decade, low off-state currents and good saturation. Low and tunable threshold voltages of 1-2 V were achieved. We conclude that a charge trapping mechanism at the semiconductor/dielectric interface is responsible for the threshold voltage shift after a gate bias stress. The threshold voltage is recovered when the bias is removed.

Optimal composition of europium gallium oxide thin films for device applications

Europium gallium oxide (EuxGa1-x)2O3 thin films were deposited on sapphire substrates by pulsed laser deposition with varying Eu content from x=2.4 to 20 mol %. The optical and physical effects of high europium concentration on these thin films were studied using photoluminescence (PL) spectroscopy, x-ray diffraction (XRD), and Rutherford backscattering spectrometry. PL spectra demonstrate that emission due to the D50 to F7J transitions in Eu3+ grows linearly with Eu content up to 10 mol %. Time-resolved PL indicates decay parameters remain similar for films with up to 10 mol % Eu. At 20 mol %, however, PL intensity decreases substantially and PL decay accelerates, indicative of parasitic energy transfer processes. XRD shows films to be polycrystalline and beta-phase for low Eu compositions. Increasing Eu content beyond 5 mol % does not continue to modify the film structure and thus, changes in PL spectra and decay cannot be attributed to structural changes in the host. These data indicate the optimal dop...

An Amorphous Indium–Gallium–Zinc–Oxide Active Matrix Electroluminescent Pixel

In this study, an active matrix pixel was fabricated and characterized using indium gallium zinc oxide (IGZO) thin-film transistors and a novel electroluminescent (EL) Eu:IGZO thin-film phosphor. The results show that even large and unoptimized IGZO devices are capable of modulating at the frequencies necessary for modern display technology. Furthermore, we demonstrate a rare-earth doped amorphous-oxide semiconductor (AOS) EL phosphor that can be modulated via a TFT.

Origins of parasitic emissions from 353 nm AlGaN-based ultraviolet light emitting diodes over SiC substrates

The effects of p-GaN capping layer and p-type carrier-blocking layer on the occurrence of parasitic emissions from 353 nm AlGaN-based light emitting diodes (LEDs) have been investigated. LEDs without a p-type Al0.25Ga0.75N carrier-blocking layer showed a shoulder peak at ~370 nm due to electron overflow into the p-Al0.10Ga0.90N cladding layer and subsequent electron–hole recombination in the acceptor levels. Broad emission between 380 and 450 nm from LEDs having a p-GaN capping layer was caused by luminescence at 420 nm from the p-GaN capping layer, which was optically pumped by 353 nm UV emission from the quantum wells. Broad, defect-related luminescence centered at ~520 nm was emitted from the AlGaN layers within the quantum wells.

Planar ZnO ultraviolet modulator

A planar electroabsorption modulator suitable for spatial light modulation has been constructed. The device operates near the band edge of zinc oxide at 3.3eV and is based on broadening and shifting of the unconfined exciton with an externally applied electric field. The ZnO active layer was deposited on an aluminum/titanium oxide dielectric on an indium tin oxide conducting layer on glass. A transparent conductive InGaZnO layer on a spin on glass insulator served as the top contact, allowing high electric fields to be applied transverse to the ZnO layer. The modulator operates at room temperature in transmission mode with +45% modulation at 373nm and −18% modulation at 380nm at 140V applied bias, corresponding to ∼450kV∕cm electric field across the ZnO active layer.